Class L fuses are used in high-power applications. As an example, Class L fuses are typically specified for service entrance equipment, switchboard mains and feeders, distribution equipment, and motor control centers. Class L fuses provide effective branch-circuit protection for large motors, and can be used for short-circuit isolation of fire pump circuits. Class L fuses are relatively large and heavy, and may have voltage ratings between 300 and 600 volts. Such fuses may have an interrupting rating of 200,000 amperes rms symmetrical, and an ampere rating of about 600 to 2000 amperes or more.
Current Class L fuses include a pair of conductive elements at the opposite ends of the fuse. Several fusible elements are secured to and make electrical contact with each of the opposed conductive elements. A generally cylindrical housing encloses the fusible elements. Sand is placed within the generally cylindrical housing, and this sand acts as an arc-quenching medium. Upon meeting certain minimum standards, Class L fuses are listed under UL Standard 198C promulgated by Underwriters' Laboratories, Inc. (hereinafter "UL").
Class L fuses prior to the present invention had several drawbacks. First, no Class L fuse existed which had an equally high UL rating for alternating current (AC) and direct current (DC). For example, a 600 volt, 1200 amp AC rated fuse typically had a DC rating of 300 volts. The reasons for this are not entirely known. It appeared, however, that sand placed within the interior of prior art Class L fuses was adequate only for quenching the arcs generated by high voltage AC currents, and not those created by high voltage DC currents.
A Class L fuse having both a 600 volt AC and a 600 volt DC rating would be desirable. Such a fuse could lower the number of fuse models made by fuse manufacturers and the inventory requirements of these relatively expensive fuses for both manufacturers and users.
A second drawback was that no known prior Class L DC fuse was UL rated at 600 volts. The highest rated Class L fuse previously known had a 500 volt rating.
A third drawback of prior Class L fuses concerned their performance under overload conditions. As stated above, current Class L fuses include sand surrounding the fusible elements and within the cylindrical fuse housing. This sand is intended as an arc-quenching medium. Nevertheless, arcs formed under certain severe conditions result in failure in some UL-listed Class L fuses. For example, arcs formed within such Class L fuses generally begin at or near the center of the fusible element, and then move quickly towards the opposing ends of the fuse. The sand would not fully quench such arcs, and the arcs would reach the inboard circular, disc-shaped end walls of the conductive elements. These arcs could literally eat away at the inner portion of these end walls and, under extreme conditions, create holes in the end walls. Obviously, this result is highly undesirable, as fuses are typically designed to safely contain any arcs, and prevent a rupture or breach through a fuse wall or through any other fuse structure. If a portion of a fuse wall or other structure is breached, parts of the contents of that fuse could be released into the surroundings. Such release carries the potential for harm to personnel and adjacent electrical devices.
Several possible solutions to these problems were contemplated, but abandoned. For example, Class L fuses include multiple, elongated, fusible elements spaced around the longitudinal axis of a fuse. A rubber disc acting as an arc barrier was proposed for placement at each inner end of the fuse, with slots for the passage of the fusible elements through that disc. After further consideration, problems were anticipated and this idea was rejected for several reasons.
First, these passage slots for the fusible elements could grow over time or be initially oversized. As a result, the integrity of the intended arc barrier-forming seal between the disc and fuse elements could not have been assured. Sand or other fine pulverulent material within the fuse interior could become wedged in these slots, compromising the arc barrier.
Second, a one-piece, slotted rubber disc would have to be placed over the fusible elements prior to soldering those elements onto the end walls. As a result, heat from the soldering process could have been transferred to and melted or distorted a portion of the rubber disc. The resulting seal between the interior of the fuse and the end walls of the fuse could have been compromised. Also, inserting the individual fusible elements into the disc slots would be a tedious and costly procedure.
Relevant prior art includes the above-described prior art Class L fuses, and also U.S. Pat. No. 4,636,765 (hereinafter "'765 patent"), issued to Littelfuse, Inc., the assignee of the present application, on Jan. 13, 1987. This patent is entitled "Fuse With Corrugated Element," and is directed to a fuse having a plug of initially solid, arc-quenching material 32. This arc-quenching material fills only one end of the fuse and is designed to evaporate under fuse blowing conditions. In addition, the arc-quenching material of the '765 patent surrounds only one fine, cylindrical fuse element 23. The arc-quenching material 32 disclosed may be selected from materials including thermoplastic polyamide polymers and polymerized fatty acids and silicates, such as those manufactured by the 3M Company, St. Paul, Minn., and sold as adhesives under Stock Nos. 3779 and XG-3793. The '765 patent does not suggest the use of silicone rubber-like materials for arc-quenching. These silicone rubber-like materials are the preferred arc-quenching materials in the present invention. The '765 patent also fails to suggest that the use of these materials may increase the DC voltage capacity of a Class L fuse. Further, the '765 patent does not teach the use of these materials in the manner of the present invention. Particularly, the '765 patent fails to teach the filling of gaps between the various fusible elements arranged around the longitudinal axis of the fuse.